[Twisted-Python] Plugins in files with hyphenated names found in 2.5.0 but not in 8.1.0

[Terry Jones]

We just upgraded to Twisted 8.1 from 2.5 and discovered that
twisted.plugin.getPlugins no longer finds our plugins.

After much head scratching, Esteve noticed that the plugins were not being
returned because the are in files that have hyphens in their names. There's
no mention of this in the release notes, at least that I can see. Perhaps
the change crept in unnoticed.

I guess this restriction on plugin-containing filenames should either be
documented, or the code should be made to work as it did. I'm happy to open
a ticket - though the text of the ticket would depend on which solution is
preferred.

Terry